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Dust devils pay visit

to PhoenixBY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: September 12, 2008

After three months of performing experiments in the north polar regions of Mars, NASA's Phoenix Lander has finally seen dust devils, and sensed a dip in air pressure as one passed nearby.

"We expected dust devils, but we are not sure how frequently," says Phoenix Project Scientist Leslie Tamppari of NASA's Jet Propulsion Laboratory. "It could be they are rare and Phoenix got lucky. We'll keep looking for dust devils at the Phoenix site to see if they are common or not."

At least six different dust devils, ranging in diameter from about two metres to five metres, appear in the images captured this week, much smaller than dust devils that NASA's Mars Exploration Rover Spirit has photographed closer to the equator.

Click for animation. The Surface Stereo Imager caught this dust devil in action on 9 September. It was about 1 km away from the Lander when the first frame was taken, ,moving to about 1.7 km away two and a half minutes later. Image: NASA/JPL-Caltech/University of Arizona/Texas A&M University.

"It was a surprise to have a dust devil so visible that it stood with just the normal processing we do," says Mark Lemmon, lead scientist for the stereo camera that captured the dust devils in action. "Once we saw a couple that way, we did some additional processing and found there are dust devils in 12 of the images. It will be very interesting to watch over the next days and weeks to see if there are lots of dust devils or if this was an isolated event."

As well as performing science experiments within the onboard laboratory, Phoenix also monitors the daily air pressure, and on the same day the camera saw dust devils, the pressure meter recorded a sharper dip than ever before seen. Phoenix also recorded wind speeds of at least five metres per second.

"Throughout the mission, we have been detecting vortex structures that lower the pressure for 20 to 30 seconds during the middle part of the day," says Peter Taylor of York University, Toronto, Canada, a member of the Phoenix science team. "In the last few weeks, we've seen the intensity increasing, and now these vortices appear to have become strong enough to pick up dust."

Phoenix scientists believe that the whirlwinds are getting stronger as the difference in day and nighttime temperatures increases. While daytime highs at the Phoenix site are still about minus 30 Celsius, the nighttime lows have been dropping steadily as summer in the northern hemisphere draws to a close, getting close to minus 90 Celsius.

The decline in daylight hours means that the project scientists must hurry to squeeze every last ounce of science out of the mission before Phoenix is unable to produce the electricity required to dig and perform experiments, which it does via its solar panels. With that in mind, a sample of icy soil from the Snow White trench has been selected for the final cell in the wet chemistry laboratory, and plans are being made to fill the remaining four of eight single-use ovens in the Thermal Evolved and Gas Analyser, without waiting for analysis of each sample to be completed before delivering the next.

"Now that the sun is not constantly above the horizon at our landing site we are generating less power every sol," says Phoenix Project Manager Barry Goldstein. "When we landed in late May, and through much of our mission, we generated about 3,500 watt-hours every sol. We are currently at about 2,500 watt-hours, and sinking daily.”

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